You can toggle it on (once the 3.5.14 update gets pushed out to your phone) by going to Menu>Settings>Voice>"Ok Google" Detection. It seems that Google is enabling the feature phone by phone though, so there's no guarantee it's coming to yours right this second.

The new version of the Google Search app also has an option for "Voice History," which will keep track of your voice commands to help make Google Now better at recognizing your lovely, melodic queries.

The slightly creepier angle is that this means your phone is listening to you more than ever, but you can just not enable it if you are afraid of the future. As for me, I can't get this update fast enough. [Droid Life]